BR112012021237B1 - rotational molding apparatus and method for producing plastic articles by rotational molding technique - Google Patents

Abstract

method and apparatus for rotational molding. The present invention relates to a process and apparatus for producing plastic articles by the rotational molding technique and is more specifically directed to a process and an oven rotational molding apparatus for producing layered containers of the materials to be melted. in the molds, which are controlled through a computerized control panel.

Description

“ROTOMOLDING APPLIANCE AND METHOD TO PRODUCE PLASTIC ITEMS THROUGH THE ROTATIONAL MOLDING TECHNIQUE”

FIELD OF THE INVENTION

The present invention relates to a process and an apparatus for producing plastic articles using the rotational molding technique and is more specifically concerned with an oven rotational molding process and apparatus for the production of multilayer containers

BACKGROUND OF THE INVENTION

Rotational molding is a technique for the manufacture of hollow articles such as containers from a thermoplastic material. The particles of this thermoplastic material in liquid, powder or granule form are placed inside a hollow mold and the mold is rotated in a hot environment until the thermoplastic material melts on the entire interior surface of the mold.

There are several types of machines, from small and simple, to others with large circulation diameters with a sophisticated electrical control system. Their selection is made based on certain parameters to be considered, such as the size of the part, the production rate, the expected inversion.

There are also different rotational molding systems; the simplest ones known as fixed or static in which the mold carrier cars enter and leave; others with a central rotating unit or carousel, with three, four or more arms that move between different work stations (loading, heating, cooling, loading / unloading). For large parts, rotary shuttle machines are used.

The traditional rotational molding process is carried out with an open flame, with the disadvantage that the thickness of the walls of the resulting article is not homogeneous due to the fact that the mold heating is irregular. On the other hand, rotational molding processes in the oven allow to obtain high quality articles with homogeneous thickness walls. Even so, when it comes to producing articles with multiple layers, the use of an oven is impractical, since once the first layer is formed, the next load of material to be melted usually causes the mold to be removed. the oven. As an alternative, to avoid having to remove the mold from the oven, open a hatch from the oven and introduce the thermoplastic material either by means of a container or box usually called “drop-box” or by means of a sprinkler device. In general, the feeding step, especially when using the “drop-box”, must be performed manually and in some cases it is necessary to suspend the heating, since the material inside the “drop-box” can plasticize before having been poured into the mold. The foregoing implies delays in the overall process, loss of heat from the oven and consequently greater energy expenditure. The feeding stage Petition 870190077981, of 12/08/2019, p. 11/25

2/10 tion becomes, however, more limiting, when it comes to manufacturing several articles of different layers in a single cycle.

In the state of the art there are several devices that have the function of introducing materials into the rotational molding furnace. US patent 4,687,531 to Potoczky describes a device for forming structures with uniform walls that are produced by means of rotation and heat in a mold, the mold being in a horizontal position; to achieve its objective the device of the state of the art uses a device to introduce the material with a material sprayer, which by means of compressed air spreads the material with the use of an arm, the end of which is inserted into the mold, being able to this arm moves around a horizontal axis and the part from which the materials leave has a 90 ° movement, the mold being found in an oven that is heated to a desired temperature. This device has the disadvantage that, when placing the material, there are places where the placement of the material does not take place, in addition, a person who knows how to maneuver the arm and place the materials in a uniform manner is necessary, whereas the device of the present invention makes use of molds that can be reused immediately to introduce the same material into the next mold; this is advantageous in a production line of more than one mold, moreover, for the formation of two or more layers in the device of the prior art it is necessary to place another arm with some other material, which incidentally is not suggested in it; finally, the arm has no means of cooling that facilitate the introduction of a different material to form two or more layers.

US patent 3,744,951 to Szatkowski describes a system for producing containers, the system consists of an oven with carousel-shaped containers, it has different stations in which the molds pass in each stage of formation, a station has an arm for the introduction of the material, the arm is introduced into the molds to spread the material in powder form and then be removed to continue the rotomoulding, although this is an arm that enters and exits the mold it has the disadvantage that the molding is carried out sprinklers using a large number of auxiliary elements, in addition, to move the arm it is necessary to move it with the entire base set, with consequently significant energy consumption; moreover, for the formation of two or more containers, it is necessary that each mold moves to the position of the arm, which causes a smaller number of containers to be produced per unit of time, because when using a carousel, it can only feed a single vessel and for the realization of two or more layers it would be necessary to have two or more arms with material which would increase the cost of production, since the arm can only be used for a single material. The disadvantage of this device is that it is more maintained and that you can only insert one material at a time. Cleaning at each introduction is not possible unless the production line is interrupted.

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The publication wo 2008/133535 shows an oven in which molds of different aspects are introduced and in which the rotation is made around the horizontal axis to produce parts, this device having a door in which there is a window and into which a tube is inserted. that by means of pressure, the dust of the material is spread inside, which comes into contact with the hot mold and forms the desired part; however, this furnace only produces a single piece at a time and it is not mentioned that the material is deposited and that due to the rotation it is evenly distributed in the mold. The material is inflated into the mold and not only is deposited for later, through rotation, it is distributed in the mold. The disadvantage of these devices is that, to make two or more layers it would be necessary to have an arm for each material since it cannot be cleaned during the production of the product and it would be necessary to have two tubes that introduce the materials at a time , increasing the cost of production.

As an alternative, US patent 4,632,654 describes the use of an oven for rotational molding, which is connected to a cooling chamber, where the molds are transferred from the oven to the cooling chamber, and once loaded with the second layer material, go back to the oven and then to the cooling chamber, and so on. This system is complex, as a device is required to transfer the molds from one chamber to another, in addition to the fact that the production time is considerably extended.

Until now, no process allows the production of articles of many layers in the oven through an automated cycle, highly efficient with large production volumes and without losses of energy in which the containers are used to introduce the material in the molds and these are the same containers used in the same production cycle to place the material forming the first layer in the molds; these same containers, once cleaned, are used to place the next layer of material and so on until the desired number of layers is completed.

SUMMARY OF THE INVENTION

The process and apparatus of the present invention can be used to manufacture containers and other hollow articles from a thermoplastic resin material that is capable of forming a film under the influence of heat. Examples of such materials are polyethylene and other polyolefins, such as, for example, polystyrene and PVC, or engineering plastics.

In accordance with the present invention, the thermoplastic material is deposited directly into one or more molds within the mold chamber. The feeding system proposed in the present invention consists of a multiplicity of containers that are introduced through gates formed in the walls of the oven, without the need to remove the molds from the molding chamber. The containers have a system 870190077981, from 12/08/2019, p. 13/25

4/10 m of cooling that prevents the thermoplastic material from adhering to the surface of the container and they are also subjected to a cleaning step before carrying out the following mold loading, which prevents the layer (s) from being contaminated with material from the previous load.

The present invention furthermore proposes an extremely efficient automated feeding system, ideal for processes involving several molds within the same production cycle. This system is based on the simultaneous feeding of two molds through the introduction of two containers that contain the thermoplastic material in the oven, then the placement of two other molds and the introduction of two other containers with material to be melted and so on. successively, until all the molds have the charge of the thermoplastic material. Then, the molds are rotated within the molding chamber to form a first layer. Then the material that will form the second layer is fed, using the feeding method already described and the rotation of the molds to form the second layer, and so on. It is important to note that the oven is at a virtually constant temperature throughout the production cycle, which is an energy and economic advantage.

The present invention is an improvement on the prior art, as it allows multiple-layer articles to be produced in an oven with a molding chamber, by means of an extremely efficient process, without delay in the step of feeding the thermoplastic material, without wasting energy, obtaining high quality articles and large production volumes in a short time, using a series of containers to introduce the materials to be melted into the molds, without the need to use spray or injection means to dose the material thermoplastic.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a perspective view of the rotational molding apparatus of the present invention.

Figure 2 is a side view of the rotational molding apparatus of the present invention.

Figure 3 is a perspective view of the furnace that also shows the floodgates through which the material to be melted is introduced.

Figure 4 represents a plan view of one of the plates that support the molds.

Figures 5a and 5b are a perspective view of one of the containers that transport the material to be melted in the oven, the container being attached to a rotating arm; Figure 5a shows the container in position to contain the material to be melted and Figure 5b shows the container in position to pour its contents.

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Figure 6 is a top perspective view of one of the containers from which the top bottom has been omitted.

Figure 7 is a top plan view of the container holding the thermoplastic material, without the top bottom.

Figure 8 is a perspective view of the container cooling system.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows a perspective view of the system of the present invention, which consists mainly of an oven (1) with a molding chamber and a feeding system for the thermoplastic material, adjacent to the oven.

The oven (1) is a thermally insulated structure, preferably with steel sheet panels. In the preferred mode, the oven has an octagonal shape, as this configuration allows greater efficiency in heating. However, chambers of any other polygonal shape, such as, for example, hexagonal, can be used, including chambers of cylindrical, spherical, cubic or other parallelepiped shape.

The oven (1) has heating means (not shown in the Figures) that raise the temperature of the chamber sufficiently to plasticize the thermoplastic material. The preferred heating means are selected from burners powered by methane gas or LP gas (liquefied propane gas), digital burners, by electric arc, among others.

As seen in Figure 3, the oven (1) has doors for opening / closing the oven (not shown in the Figures) and, in addition, one or more floodgates (8), (9) formed on at least one of the faces sides of the oven, which preferably have a door to close and open the gate to avoid thermal losses; these penstocks allow the material to be plasticized into the oven chamber. In the preferred embodiment, the oven has two side gates (8) located at the same height and also has a central gate (9) that will be used to manufacture larger tanks. It is preferable that the penstocks (8) and (9) are at an intermediate height in relation to the length of the oven chamber.

The oven (1) contains a mold holding means, traditionally known as a spider. In the preferred embodiment, the spider consists of two plates, a front plate (3) and a rear plate (3 ') (see Figures 1 and 2). In place of the plates, other support units can be used, such as, for example, frames with a multiplicity of arms. The plates (3), (3 ') are coupled in their center to an arrow (13) placed on the “X” axis as shown in Figure 2. The plates are placed opposite each other, so that their external faces are free to receive and support the molds (2).

The rotating arrow (13) in turn is coupled perpendicularly to another

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6/10 arrow (30) that transmits the rotation movement to the spider around the “Y” axis. The arrow (30) is shown in dashed lines in Figure 1. The arrow (30) allows the spider to be rotated 180 ° to transfer the molds (2) of the front plate (3) back and the molds (2) of the rear plate (3 ') forward. With the described arrangement, each mold (2) is capable of receiving the material to be melted through the gates (8), (9).

In the preferred embodiment, the number of molds (2) is eight, four placed on one plate and the other four on the other plate. However, in the case of small molds, the plate can support a larger number of molds. Similarly, the use of larger molds will involve fewer of them. You can even choose a single mold on each side of the spider, depending on the needs of the production. In this case, the introduction of the material to be melted can be carried out through the central gate (9).

The thermoplastic material that is introduced into the molds (2) comes from one or more feeder containers (4) each feeder container (4) consists of a tray that receives and contains the material to be melted. The container (4) is connected at its end to an arm (5) which in turn is coupled to a reducing motor (6), transmitting this motor (6) a 180 ° rotation movement to the arm to spill the container (4) and thus pour the material to be melted into the mold. Each of the arms (5) is also coupled to a second reducing motor (16) that transmits the movement of the arm (4) forward and to introduce the container (4) through one of the gates (8) or (9) and back to remain in line with the rest of the arms (4).

In the preferred embodiment, the number of containers (4) corresponds to the number of molds (2), so that each container (4) pours its contents into a specific mold. The eight containers (4) are placed at the same height and are aligned with each other, as shown in Figure 1. The containers are furthermore grouped into two groups of four, so that four containers (4) are close to each other. one side gate (8) and the other four containers (4) are close to the other side gate (8). This configuration allows a container (4) of a first group to be introduced through a gate (8) and the other container (4) of the second group through the other gate (8). The ideal is to introduce the two containers (4) simultaneously.

The arms (5) with their respective containers (4) also have a horizontal displacement that is obtained by means of a third reducing motor. This displacement allows the positioning of all containers to be able to unload their contents in the molds as needed during the global process.

As will be evident to those skilled in the art, the movement of the containers (4) is not limited to the use of reducing motors, but hydraulic, mechanical, pneumatic, manual means can also be used.

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In the following the container (4) of the present invention will be described. It is preferable that each feeder container (4) has rounded walls, as seen in Figures 5a and 5b, to facilitate the leakage of material in the mold and to prevent residual material from adhering to the container (4).

The feeder container (4) must be cooled to prevent particles from adhering to it, since the hot container can easily allow the plastic material from the previous discharge to adhere. The present invention proposes a cooling system by means of water or another cooling liquid, circulating the cold liquid from a liquid cooling equipment to each of the containers (4) as will be explained below, making reference to Figures 6 and 7. It is preferable that the coolant is water, which offers the advantage of being recyclable, of low cost and high heat capacity.

The container (4) is a structure with a double bottom, so that a chamber is formed through which the water can circulate (see Figure 6). The container has a cold water inlet (20) and a hot water outlet (21). The holes (20), (21) are preferably formed at one end of the container, with even more being preferred at the front end. The container (4) has a longitudinal division (22) formed inside the chamber, this division (22) extending from the front end of the container (4), but ending before the rear end. This division (22) allows the water that penetrates through the inlet hole (20) to follow a trajectory similar to a “U” (see Figure 7) and then out through the hole (21).

It should be mentioned that water can be circulated inside the container chamber by other means, such as with an internal pipe, a coil etc., or any other element that allows the flow of water to perform the cooling, however it is preferable to use the vertical division (22) due to its simplicity and effectiveness.

As shown in Figure 8, the water that enters the containers (4) comes from a cooler (23) that sends cold water to a first cold water distributor (24), with this distributor (24) in turn hoses ( 25) connected that transport cold water to the containers (4) through the inlet holes (20). The hot water comes out of the outlet holes (21) through second hoses (26) that transport the water to a second distributor (27) that returns the hot water to the cooling equipment (23).

The hoses (25), (26) are preferably produced from a heat resistant material, as they are always connected to the containers and will also be introduced into the oven during the stage of unloading the molds. Similarly, the hoses (25), (26) must be made of a flexible material that allows them to adapt to the rotation and displacement of the containers (4).

It is preferable that the containers are made of stainless steel covered with ma

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8/10 non-stick material or with another release material, chrome iron, steel or carbon, aluminum or any other material resistant to high temperatures.

In the preferred embodiment of the present invention, considering four molds (3) on each plate (3), (3 '), the position of the moles is as shown in Figure 4, so that two of them are on the horizontal axis and the remaining two on the vertical axis. The spider has two positions for feeding the molds, these positions being determined by the rotation transmitted by the arrow (13). In a first position, two molds are simultaneously fed, those found, for example, on the horizontal axis, by means of the introduction of two containers (4). Then, the plate is rotated 90 ° to feed the remaining two molds. Then, the spider rotates 180 ° by means of the arrow (30) to position the molds of the rear plate (3 ') in front of the floodgates. The loading operation is repeated as with the first plate (3) for the remaining molds. The procedure is repeated to form each of the layers that the product requires.

An important advantage of the present invention consists in the fact that after the container (4) is cold, there is no residue adherence to the walls of the container, avoiding contamination of the subsequent layers. However, a pressure cleaning system is also required to remove particles from the previous discharge and the container is ready for the other charge. Compressed air is applied to the containers when they leave the oven. The container cleaning system is preferably placed on the support of the feeding system, under the containers. It consists of a venturi system, preferably of the fan type to cover most of the container. Any device with air movement can be used in its place. When you want to clean the containers, they are turned down to receive the compressed air. It is preferable that the residue of the containers is placed in a container.

The weighing and dosing system for the thermoplastic material proposed for the preferred modality is already known. It consists of one or more vacuum cleaners (10), each vacuum cleaner (10) connected to a hopper (11) and a gravimetric scale (12) that has electronic level indicators. The discharge into the containers (4) is done through an opening system. The vacuum cleaner (10) absorbs the plastic material from a reservoir (not shown in the Figures) and transfers it to the hopper (11). The gravimetric scale (12) electronically sends a signal to the hopper (11) to interrupt the feeding according to the weight for which the scale is programmed.

In the preferred embodiment, to produce a three-layer article. Three hoppers, three vacuum cleaners and three scales were used. The system is configured so that, during the production of the first layer, a single scale (12) feeds all containers with the first material to be used. For the second layer, the second hopper will be used, and so on.

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Alternatively, the containers can be fed manually, pneumatically, hydraulically, among others.

The system of the present invention has an oven control panel (40) and a power control panel (41), both computerized systems. The feed control panel works by coordinates. Both the supply and control systems of the furnace are synchronized for material feeding.

The system of the present invention also contains a support device for the feeders and a support for the arms and containers.

As will be apparent to those skilled in the art, multiple modifications can be made to the system described in the present application. The movement of the molds inside the oven, for example, can be carried out in different ways, in the form of a carousel, for example.

The process of the preferred embodiment of the present invention consists of the following basic steps:

1. Load the hoppers (11) and gravity scales (12) with the programmed amount of material to be melted.

2. Load the containers (4) with the material from one of the hoppers (11).

3. Place the molds (2) of one of the plates (3) in front of the floodgates (8).

4. Open the floodgates (8).

5. Transfer a container from the first group of containers to the oven through one gate (8) and simultaneously transfer another container from the second group of containers through the other gate (8).

6. Rotate the two containers (4) simultaneously to pour their contents into the molds (2) adjacent to the penstocks (8)

7. Rotate the front plate (3) around the “X” axis to place two empty molds (2) in front of the floodgates (8)

8. Repeat steps 5 and 6 with the empty plate molds (3).

9. Rotate the plates (3) and (3 ') around the “Y” axis by means of the arrow (30) to place the rear plate (3') in front of the floodgates (8).

10. Repeat steps 3 to 8 to load the molds (2) of the rear plate (3 ').

11. Rotate the molds inside the molding chamber until the first layer is formed.

12. Repeat steps 2 to 11 for the subsequent formation of the other layers of thermoplastic material.

13. Open the oven and remove the molds and remove the finished parts.

It should be added that the cleaning step of the containers (4) with compressed air can be carried out either at the moment when the containers leave the oven or immediately

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10/10 before placing the material from the hopper.

As can be seen from the previous description, the production cycle for plastic articles is fully automated. Another advantage of the procedure proposed here is its great versatility, since articles of different shapes and sizes can be obtained simultaneously and with the same machine, and even having different thicknesses.

The product resulting from this process has all the advantages of rotational molded articles in the oven, since the homogeneity of the thickness of the walls is preserved, obtaining high quality articles. The resulting product has improved attributes and lower manufacturing costs.

Another advantage is the low energy cost compared to the processes that require the extraction of the molds from the oven for a new face or other processes in which the oven is opened to manually introduce the load of the material to be melted, since each opening the oven implies heat loss. For this reason, the process is also more economical and has no impact on the environment.

With the apparatus and method proposed here it is possible to save up to 50% of time than would be necessary with the practice of the traditional way. The synchronization of the oven with the feed in an automated way by means of coordinates results in a saving of time such that it reduces the production time.

In the preferred embodiment, the product resulting from the process is a three-layer tank, the outer layer being a solid material based on polyethylene, the intermediate layer being a foam based on polyethylene and the inner layer of an antibacterial material based on polyethylene .

It is evident that the modalities of the appliance components described in this application can be made, for example, with the use of a greater number of penstocks to perform simultaneous feeding of all the molds inside the oven, however the system becomes more complex. On the other hand, the use of a coordinate system allows the execution of multiple mold feeding with only two locks, without the need to open the main gate. In addition, the fact that an intermediate door is used for larger molds is also evidence of the versatility of the device described here.

Claims (5)

1. Rotational molding apparatus consisting of an oven (1) with a molding chamber and a feeding system, the oven (1) having means to hold the molds (2) inside which a thermoplastic material, heating means and means that transmit the rotating movement to the molds (2), CHARACTERIZED by the fact that: a) the oven has one or more gates (8, 9) formed on one of the side sides of the oven, each of the gates (8, 9) being designed to allow the introduction of at least one container (4) in the oven, consisting of the container or containers in structures carrying the thermoplastic material, located outside the oven (1) and adjacent to the gate (s) (8, 9), each container being connected to an arm (5) which has (i) means to move the container (4) back and forth in order to insert and remove it through one of the gates (8, 9); and (ii) means for rotating the arm (5) for the purpose of spilling the container (4) and pouring its contents into the molds (2) inside the oven (1); b) each container (4) is a double bottom structure so that, between its two walls, a chamber is formed, through which a liquid from a cooler (23) is circulated, the cold liquid penetrates through a inlet orifice (20) formed in the container (4), the cold liquid circulates through the chamber of the container (4) and the hot liquid exits through an outlet orifice (21). 2/4 CHARACTERIZED by the fact that each container (4) has a longitudinal division (22) to allow a trajectory of the cooling liquid in a “U” shape. 9. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the means of introducing and extracting the containers (4) in the oven (1) consist of one or more reducing motors (6). 10. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the number of penstocks (8) is two. 11. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the number of penstocks (8, 9) is three. 12. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that there is a central gate (9) that will be used to manufacture larger tanks. 13. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the locks (8, 9) are located at the same height. 14. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the gates (8, 9) are at an intermediate height in relation to the length of the oven chamber (1). 15. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the containers are controlled by means of a computerized control panel (40, 41). 16. Rotational molding apparatus, according to claim 1, CHARACTERIZED by the fact that the apparatus has a support device for the feeders and a support for the arms (5) and the containers (4). 17. Rotational molding apparatus according to claim 1, CHARACTERIZED because it also includes means for cleaning the containers (4). 18. Rotational molding apparatus according to claim 17, CHARACTERIZED by the fact that the cleaning means consist of one or more means to send compressed air. 19. Rotomoulding apparatus, according to claim 1, CHARACTERIZED by the fact that it also comprises a cooling system configured to cool by means of water or other cooling liquid. 20. Rotational molding apparatus according to claim 19, CHARACTERIZED by the fact that the cooling system uses one or more designed hoses (25) to circulate the cooling liquid to the containers (4). 21. Rotational molding apparatus according to claim 20, CHARACTERIZED by the fact that the hoses (25) circulate the cold liquid from a cooling equipment (23) to each of the containers (4). Petition 870190077981, of 12/08/2019, p. 22/25 2. Rotational molding apparatus according to claim 1, FEATURED by the fact that the containers (4) have rounded walls. 3/4 22. Rotational molding apparatus according to claim 21, CHARACTERIZED by the fact that the cooling equipment (23) is in turn coupled to a first distributor (24) which in turn transfers the cooling liquid to the hoses (25 ). 23. Rotomolding apparatus, according to claim 22, CHARACTERIZED by the fact that the liquid that comes from the containers (4) passes to a second distributor (27) which in turn transfers the hot liquid to the cooling equipment (23) . 24. Method for producing plastic articles using the rotational molding technique using a rotomoulding apparatus as defined in claim 1, CHARACTERIZED for comprising the following steps: a) load some hoppers (11) and some gravity scales (12) with the programmed amount of material to be melted; b) load some containers (4) with the material from one of the hoppers (11); c) place some molds (2) of one of the plates (3) in front of the floodgates; d) open the floodgates (8); e) transferring a container (4) from the first group of containers to the oven (1) through a gate (8) and simultaneously transferring another container (4) from the second group of containers through the other gate (8); f) simultaneously rotate the two containers (4) to pour their contents into the two molds (2) adjacent to the penstocks (8); g) rotate the front plate (3) around the “X” axis to place the two empty molds (2) in front of the floodgates (8); h) repeat steps e) and f) with the empty molds (2) of the plate (3); i) rotate the plates (3, 3 ') around the “Y” axis by means of a spear (30) to place the rear plate (3') in front of the floodgates (8); j) repeat steps c) to h) to load the molds (2) of the back plate (3 '), k) rotating the molds (2) inside the molding chamber until the formation of the first layer; l) repeat steps b) to k) for the subsequent formation of the other layers of thermoplastic material; and m) open the oven (1) to remove the molds and extract the finished parts from the molds. 25. Method for producing plastic articles using the rotational molding technique, according to claim 24, CHARACTERIZED by the fact that the containers (4) are cleaned with compressed air, which can be done or at the moment when the containers (4) come out of the oven (1) or just before placing the material from Petition 870190077981, of 12/08/2019, p. 23/25 3. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the weighing and dosing of the thermoplastic material is carried out by means of a gravimetric scale (12). 4/4 between the hopper (11). 26. Method for producing plastic articles using the rotational molding technique, according to claim 24, CHARACTERIZED by the fact that the method is implemented automatically. 4. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the rotating arm of the containers rotates by the action of a reducing motor (6). 5. Rotational molding apparatus, according to claim 1, CHARACTERIZED by the fact that the container has a longitudinal division (22) that allows the liquid that enters through the inlet hole (20) to follow a trajectory similar to a "U" . 6. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that each gate (8, 9) has means of opening and closing. 7. Rotational molding apparatus according to claim 1, CHARACTERIZED by the fact that the means for making the container (4) rotate perform a 180 ° rotation. 8. Rotational molding apparatus according to claim 1, Petition 870190077981, of 12/08/2019, p. 21/25 5 27. Method for producing plastic articles using the rotational molding technique, according to claim 24, CHARACTERIZED by the fact that it is performed using a computerized control panel (40, 41).